The purple-backed thornbill (Ramphomicron microrhynchum) is a species of hummingbird in the "coquettes", tribe Lesbiini of subfamily Lesbiinae.

The tail is shorter than the male's and less forked, bronzy purple, and the outer pair of feathers have white tips.

R. m. albiventre is very similar to the nominate, but with lighter margins on the male's undertail coverts and smaller spots on the female's underparts.

Males of R. m. bolivianum have metallic violet upperparts, dark green underparts, and grayish white undertail coverts.

[5] The purple-backed thornbill's breeding season has not been fully defined but appears to span from May possibly to December.

The purple-backed thornbill's song is "a continuous series of quiet scratchy notes interspersed by short buzzes or dry trills."